1. Field of the Invention
The present invention relates to pumpable, highly concentrated aqueous pastes of alkali metal salts of .alpha.-sulfonated fatty acid alkyl esters and processes for their preparation.
2. Description of Related Art
It is known that .alpha.-sulfofatty acid ester salts may be obtained in the form of aqueous pastes by neutralizing .alpha.-sulfofatty acid esters, usually with an aqeuous alkali metal hydroxide. The starting materials used are natural fats and/or oils obtained by ester cleavage and subsequent esterification with lower alkanols, more especially methanol, or by transesterifying the natural triglycerides with the lower alkanols. Depending on the origin of the natural starting material, the fatty acid ester mixtures obtained contain a comparatively broad range of fatty acids, normally in the C.sub.10 to C.sub.24 range. The sulfonation of these fatty acid ester mixtures, usually with gaseous SO.sub.3, leads to more or less heavily discolored, acidic crude sulfonates which have to be bleached and converted into ester sulfonate pastes by neutralization to a pH-value in the range of from about 6 to 7. In this form they are now acquiring increasing practical significance as surfactants and wetting agents for detergents and cleaners based on renewable raw materials of natural origin.
One particular difficulty involved in handling pastes of alkali metal salts of .alpha.-sulfonated fatty acid alkyl esters (hereinafter referred to as ester sulfonate salts) is their concentration/viscosity behavior. It is only at comparatively low solids concentrations (for example up to solids contents of approximately 35% by weight) that ester sulfonate salts produced on an industrial scale form, in aqueous mixture, sufficiently fluid solutions or suspensions to guarantee the uninterrupted operation of industrial processes. At higher ester sulfonate salt contents (solids contents of around 40% by weight or higher), the viscosity of the aqueous preparation increases to such an extent that it no longer flows freely. This gives rise to serious limitations, which are discussed below.
Attempts to produce highly concentrated ester sulfonate salt pastes directly by neutralizing the crude sulfonic acid mixture with concentrated alkali metal hydroxide solutions fail because the stirrability and, hence, uniform miscibility of the reaction mixture is lost. At the same time, it becomes impossible to dissipate the heat of neutralization. Local concentration and temperature peaks give rise to undesirable secondary reactions, including in particular the formation of disalts of the .alpha.-sulfofatty acids in large quantities through ester cleavage. Another disadvantage is that, understandably, ester sulfonate pastes immobilized by an increase in viscosity can no longer be pumped on an industrial scale. Pipes become blocked and the plant as a whole is brought to a prolonged standstill.
The prior art on ester sulfonate salts pastes of this type is greatly concerned with these particular problems. In particular, it has been proposed to use flow aids and viscosity regulators to improve the flow behavior of aqueous, commercial concentrates of .alpha.-sulfofatty acid ester salts. Thus, German Application No. 33 05 430 for example describes the use of C.sub.8 -C.sub.40 alcohols which, in addition, may contain one or more hydroxyl groups as substituents and to which up to 20 moles of ethylene oxide and/or propylene oxide per mole of alkanol may be added. These viscosity regulators are added to the ester sulfonate paste in quantities of from 1 to 15% by weight, based on the quantity of surfactant, to adjust the viscosity of the surfactant concentrate to at most 10,000 mPas at 70.degree. C. According to German Application No. 33 34 517, aqueous suspensions containing from 40 to 65% by weight of .alpha.-sulfofatty acid ester salt, from 2 to 10% by weight of a lower alcohol sulfate and, optionally, at most 2% by weight of a lower alcohol are said to be sufficiently fluid. These aqueous multicomponent mixtures are obtained by initially preparing aqueous suspensions containing from 30 to 55% by weight of the .alpha.-sulfofatty acid ester salt in admixture with from 5 to 15% by weight of a lower alcohol sulfate and from 8 to 40% by weight of a lower alcohol and then concentrating the aqueous suspension to the above-mentioned concentration values. According to German Aplication No. 31 23 681, a highly concentrated aqueous solution of a salt of an .alpha.-sulfofatty acid ester may be prepared by neutralizing the crude sulfonic acid in a first stage to pH 2.5-4 with a 15 to 50% by weight aqueous caustic alkali solution in the presence of from 5 to 20% by weight of an alcohol containing up to 4 carbon atoms, adding an aqueous solution diluted to 1-5% by weight of alkali hydroxide to the still acidic partially neutralized product in a following stage and adjusting the pH-value to 6-7.